A wireless communication system includes a radio network and user devices communicating with each other via the radio network. The radio network includes base stations, which implement a bi-directional radio connection with the user devices thereby providing an access interface for the user devices using the radio network.
Examples of cellular access systems include WCDMA (Wideband Code Division Multiple Access) and a so-called 3.9G radio network. Contrary to WCDMA, 3.9G does not apply any spreading or scrambling sequences in base stations, which makes it more difficult to cope with interference from neighbouring base stations. Interference suppression thus becomes a problem for a baseband receiver algorithm. Typically a 3.9G receiver requires at least two receive antennas to make interference suppression work properly. With single antenna transmission linear receivers, such as interference rejection combining (IRC) receivers, can be used. With multiple transmit antennas, more advanced receiver algorithms are required to achieve the adequate performance. Most promising examples of these advanced receivers include iterative interference cancellers and sequence estimators operating in a likelihood-domain.
The user devices move within a network area continuously and thus the signal propagation environment with respect to the base stations changes all the time. Therefore, the quality of the radio connections between a user device and one or more serving base stations has to be monitored continuously as the optimal transmission parameters change accordingly. The most commonly used feedback information is the post-detection Signal to Interference and Noise Ratio (SINR) or a Signal to Noise Ratio (SNR) estimate, which can be used directly in a quantized form or with some other metrics to determine the set of transmission parameters that can be supported in given conditions.
Based on the feedback describing the instantaneous SINR conditions for a certain mobile terminal and based on cell loading and delay sensitivity, for instance, the base station is able to adapt its transmission parameters, such as frequency allocation, modulation or coding rate, accordingly. Such monitoring and change of signal transmission parameters is generally referred to as link adaptation.
The calculation of the SINR is rather straightforward for linear receivers, but can be fairly complex e.g. for likelihood-domain algorithms. In fact, derivation of the post-detection SINR for more advanced receivers can be too time-consuming or computationally complex, or the concept of post-detection SINR may not even be a meaningful measure.